A dielectric filter is a filter including a dielectric resonator. The dielectric resonator is formed by repetitive total reflection and resonance of an electromagnetic wave inside a dielectric. In an actual application, a dual-mode filter may be equivalent to a dual-tunable resonant circuit. A band-pass filter may be fabricated using a coupling between two modes. In comparison with a conventional single-mode filter, the dual-mode filter has many advantages such as a low loss, a small volume, an elliptic function response, and a light weight.
In the prior art, generally, there are four methods for separating degenerate modes. (1) When a coupling screw is used to implement a coupling of degenerate modes to avoid interactions, the coupling screw should be located in a region that is near to a maximum value of electric field intensity of two resonators (which need to be coupled) and in which electric field intensity of other degenerate modes is zero. Generally, there is an angle of 45 degrees between the coupling screw and two polarized electric fields. However, a tunable range of this coupling manner is relatively narrow. (2) A coupling screw is inserted from a position that is an angle of 45 degrees above a dielectric resonator, and this may also separate degenerate modes. (3) A rectangular slot is cut, but processing in this coupling manner is difficult. (4) A slot is cut in a center of a dielectric resonator, but processing in this coupling manner is also difficult.
In the prior art, there is also a subminiature band-pass filter applied to a Universal Mobile Telecommunications System (UMTS) base station. The subminiature band-pass filter includes the following several parts: two triple-mode electroplated ceramic resonators and one metallic coaxial comb resonator that is located between the two triple-mode ceramic resonators. A multi-mode principle of the subminiature band-pass filter uses the foregoing method (1) and method (3). The two resonators constitute a composite structure, which is used to solve a common problem that a parasitic characteristic of an insulated ceramic multi-mode filter is generally poor. Out-of-band suppression of the subminiature band-pass filter can achieve the following good characteristics: more than 70 decibels (dBs) if a frequency is lower than 1880 MHz and more than 95 dB if a frequency is higher than 2110 megahertz (MHz). In addition, an in-band insertion loss of the subminiature band-pass filter is less than 1 dB, and a return loss is less than −20 dBs. Resonant frequencies of three orthogonal modes of each triple-mode resonant cavity are respectively decided by a size of each side of an electroplated ceramic block. Each corner decides a coupling coefficient between two orthogonal modes. Stimulation of each triple-mode ceramic block is implemented using a probe. A size of a hole through the probe is inserted into a ceramic block decides a coupling between an input and an output. A coupling between the third mode of each triple-mode resonant cavity and a central coaxial resonator is implemented using a hole of a specific size. In subminiature band-pass filter, there is one tuning screw on ceramic blocks of three modes respectively. A response curve of the filter in an actual test is quite consistent with a response curve of a simulated filter. However, in the prior art, a shape of a ceramic body in the filter is complex, processing is difficult, and mass production is not easy. A precision of corner cutting on the ceramic body cannot be ensured, and production costs are high.